Thin film transistors (TFTs) are a type of field effect transistors (hereinafter referred to as FETs). TFTs are three-terminal elements having a gate terminal, a source terminal, and a drain terminal in the basic structure. TFTs are active elements having a function of switching the current between the source terminal and the drain terminal so that a semiconductor thin film deposited on a substrate is used as a channel layer in which electrons or holes move and a voltage is applied to the gate terminal to control the current flowing in the channel layer. TFTs are electronic devices that are most widely used these days in practical application. Typical applications of TFTs include liquid-crystal driving elements.
Currently, most widely used TFTs are metal-insulator-semiconductor-FETs (MIS-FETs) in which a polycrystalline silicon film or an amorphous silicon film is used as a channel layer material. MIS-FETs including silicon are opaque to visible light and thus fail to form transparent circuits. Therefore, when MIS-FETs are used as switching elements for driving liquid crystals in liquid crystal displays, the aperture ratio of a display pixel in the devices is small.
Due to the recent need for high-resolution liquid crystals, switching elements for driving liquid crystals now require high-speed driving. In order to achieve high-speed driving, a semiconductor thin film in which the mobility of electrons or holes, is higher than that in at least amorphous silicon needs to be used as a channel layer.
Under such circumstances, Patent Document 1 proposes a transparent semi-insulating amorphous oxide thin film which is a transparent amorphous oxide thin film deposited by vapor deposition and containing elements of In, Ga, Zn, and O. The composition of the oxide is InGaO3(ZnO)m (m is a natural number less than 6) when the oxide is crystallized. The transparent semi-insulating amorphous oxide thin film is a semi-insulating thin film having a carrier mobility (also referred to as carrier electron mobility) of more than 1 cm2/(V·sec) and a carrier density (also referred to as carrier electron density) of 1016/cm3 or less without doping with an impurity ion. Patent Document 1 also proposes a thin film transistor in which the transparent semi-insulating amorphous oxide thin film is used as a channel layer.
However, as proposed in Patent Document 1, the transparent amorphous oxide thin film (a-IGZO film) containing elements of In, Ga, Zn, and O and deposited by any method of vapor deposition selected from sputtering and pulsed laser deposition has an electron carrier mobility in the range of about from 1 to 10 cm2/(V·sec), which is relatively high. However, oxygen defects are likely to be generated in the amorphous oxide thin film, and electron carriers do not always stably behave in response to external factors such as heat. It has been pointed out that these facts often have adverse effects and cause problems associated with instability of devices, such as TFTs, when the devices are formed.
Regarding materials for solving these problems, Patent Document 2 proposes a thin film transistor including an oxide thin film in which gallium is dissolved in indium oxide. In the oxide thin film, the Ga/(Ga+In) atomic ratio is 0.001 to 0.12, and the percentage of indium and gallium with respect to the total metal atoms is 80 at % or more. The oxide thin film has an In2O3 bixbyite structure. An oxide sintered body is proposed as the material of the oxide thin film in which gallium is dissolved in indium oxide. In the oxide sintered body, the Ga/(Ga+In) atomic ratio is 0.001 to 0.12, and the percentage of indium and gallium with respect to the total metal atoms is 80 at % or more. The oxide sintered body has an In2O3 bixbyite structure.
However, there is still a problem in that the carrier density described in Examples 1 to 8 in Patent Document 2 is of the order of 1018 cm−3, which is too high for an oxide semiconductor thin film to be used in TFTs.
Patent Document 3 discloses an oxide sintered body having a bixbyite structure and containing indium oxide, gallium oxide, and zinc oxide. The quantitative ratio of zinc to indium, gallium, and zinc is more than 0.05 and less than 0.65 in terms of atomic ratio
However, in most of sintered bodies in Examples described in Patent Document 3, the zinc content ratio is as high as more than 0.10. Thin films, when formed by using the sintered bodies, have a high crystallization temperature, which causes a problem associated with difficulty obtaining a crystalline film having a bixbyite structure and having stable properties as a thin film transistor.
Patent Document 1: Japanese Unexamined Patent Application, Publication No. 2010-219538
Patent Document 2: PCT International Publication No. WO2010/032422
Patent Document 3: PCT International Publication No. WO2009/148154